Major depressive disorder (MDD) is a leading cause of total disability with inadequate treatment options and unresolved etiology. However, increasing evidence suggests that genetic and environmental vulnerabilities may converge on deficits of GABAergic transmission as a possible, causative core symptom of MDD. Other lines of research point to changes in glutamatergic transmission as being associated with MDD. In particular, subanesthetic doses of the NMDAR antagonist ketamin have rapid and lasting antidepressant effects even in otherwise drug-resistant forms of MDD, pointing to altered function of NMDA receptors. We have established GABA-A receptor gamma2 subunit heterozygous mice as an animal model with excellent construct, face and predictive validity of partially drug resistant MDD. Preliminary data show that GABA-A receptor deficits in gamma2 subunit heterozygous cultures result in markedly reduced expression and function of glutamate receptors. Treatment of mutant cultures with ketamine results in reversal of these deficits. Conversely, mice with GABA-A receptor deficit delimited to forebrain interneurons show a robust antidepressant-like phenotype. We here address the overall hypothesis that MDD is caused by reduced synaptic input from select subtypes of cortical and hippocampal GABAergic interneurons to pyramidal cells. The ensuing GABAergic deficit and altered E/I imbalance, through adaptive mechanisms results in reduced expression and function of ionotropic glutamate receptors, along with reduced functional connectivity of neurons. Transient treatment with NMDA receptor antagonists such as ketamine reverses these deficits and, following dissociation of the drug from the receptor, restores normal glutamatergic transmission. To address this hypothesis we will analyze ketamine-induced changes in expression and function of glutamate receptors and behavior in cultured neurons, brain slices and mice, respectively. We will further test whether chronic treatment with currently used antidepressants has similar effects on glutamatergic transmission. Lastly, we will use genetic deletion of the gamma2 subunit gene in small subsets of interneurons to identify interneuron subclasses that control depression-related behavior. Collectively, our proposal will contribute a major conceptual advance in understanding of the substrate of major depression as well as AD action.